A centrifugal blower used in an air-conditioning system for automobile use is, for example, disclosed in Japanese Unexamined Patent Publication No. 2004-360497. FIG. 24A is a cross-sectional view along the axial direction of a centrifugal blower of the prior art disclosed in Japanese Unexamined Patent Publication No. 2004-360497, FIG. 24B is a front view, and FIG. 24C is a plan cross-sectional view.
Such a conventional centrifugal blower is provided with a multi-blade fan 16 having a large number of blades 2, a motor 34 to an output shaft 33 of which this multi-blade fan 16 is attached, and a casing 31 housing the multi-blade fan 16 inside it and having a scroll chamber 30 formed in a spiral shape at an outer circumferential side of the multi-blade fan. The scroll chamber 30 is formed in a spiral shape which starts from a nose portion 1a of the casing 31 and gradually expands in passage toward the air outlet. In general, the center of rotation O of the multi-blade fan forms the center point of the scroll chamber. When the nose portion 1a is an arc shape, strictly speaking, the position showing the center of curvature of the nose portion 1a is the spiral start part (starting point of spiral casing). The starting point of the circumferential direction angle φ with respect to the center O is the center of curvature of the nose portion 1a. The nose portion is not limited to an arc shape. Here, the explanation will be given deeming the nose portion end part as a spiral start part.
The casing 31 has an air inlet 13 at one surface of the multi-blade fan 16 in the axial direction. When the motor 34 rotates, the multi-blade fan 16 sucks in air from the air inlet 13 to the center part of the multi-blade fan 16. The air is sucked into the center part of the multi-blade fan, then is given kinetic energy (dynamic pressure) by this multi-blade fan, has part of the dynamic pressure converted to static pressure in the casing while passing through the scroll chamber 30, and is discharged from the air outlet.
In this prior art, it is possible to reduce noise accompanying the formation of backflow near the nose portion 16. That is, the starting point 21a of the step 21 matches the spiral start part (that is, the circumferential direction angle φ (see FIG. 24C) from the spiral start part becomes 0°), while at the end point 21c of the step 21, the circumferential direction angle φ from the spiral start part becomes 10°. In this prior art, the end point 21c of the step 21 is defined as the starting point of the chamber part 35. The starting point of the chamber part 35 is φ=10°.
The prior art aimed at reduction of the noise accompanying the formation of backflow, but provided a step for sharply expanding the shape of the bottom of the scroll chamber and sharply expanded the scroll chamber passage. For this reason, a sufficient noise reduction effect could not be obtained. The “backflow phenomenon” expresses the phenomenon where part of the flow in the case enters between the blades.
On the other hand, the prior art shown in Japanese Patent No. 3231679 prevents backflow occurring largely near the nose portion by a plate and thereby suppresses a drop in blower efficiency and suppresses the generation of noise due to this backflow. FIG. 25 is a cross-sectional view of a centrifugal blower of the prior art disclosed in Japanese Patent No. 3231679.
The prior art shown in this Japanese Patent No. 3231679 provides a plate 3′ having a slanted part facing a main plate side 110 from a side plate side 109 of the multi-blade fan 16 in the axial direction of the air inlet 13 and has a maximum length part at the inside of the nose portion 1a. The distribution of peripheral speed in the direction of the electric motor shaft 33 at the outer circumference of the multi-blade fan 16 is not uniform (see Japanese Patent No. 3231679, FIG. 14), so if the blower static pressure becomes larger, backflow is formed from the side plate side 109 of the multi-blade fan 16 near the nose portion. In particular, the backflow formed large near the nose portion is prevented by the plate 3′. In this prior art, the plate 3′, set from the inlet side of the blades, is provided at the location where backflow is formed so as to solve this problem.
The plate 3′ at Japanese Patent No. 3231679, as shown in FIG. 25, is provided between the air inlet 13 and the inlet side of the blades 2 of the multi-blade fan 16 (not fan outlet side). In this case, in the backflow phenomenon in which part of the flow inside the casing enters between the blades, it was not possible to prevent entry of the backflow between the blades in advance. That is, backflow ends up entering inside the blades once, so the backflow interferes with the sucked in flow and the air flow between the blades is greatly disturbed. Therefore, worse noise and a lower flow rate (lower efficiency) are invited. Further, due to the provision of the plate 3′ set from the blade inlet side, even when backflow is not formed, suction resistance is formed and the overall flow rate is lowered to thereby cause a drop in efficiency.